Knowledge
of floristic composition and structure of forests is useful in identifying
important elements of plant diversity, protecting threatened and economic
species. Investigations into floristic composition and structure of forests are
essential in understanding
the forest ecosystem dynamics and conservation.

Using 1-ha plots established in 3 sites harboring
semi-deciduous forests in the Congo Basin, we defined floristic patterns of the
vegetation within forest layers. To account for common and rare species, we
considered the diversity indices of orders 0, 1 (exponential of the Shannon
index) and 2 (inverse of Simpson index). We used the
Chao 1 index for diversity of order 0. The jackknife and minimum variance
unbiased estimator (MVUE) were considered to estimate diversity of orders 1 and
2. We also assessed the way substrate differentiation affects species assemblages
in these forests. Results showed that floristic patterns (species composition
and species diversity) vary from a forest layer to another. Three main groups
of species assemblages were defined in the overstorey. It has been observed
that soil features (clay and sandy substrates) influence species occurrences in
these semi-deciduous forests.